The Roman army was the first army with professional soldiers in a modern context. To be a soldier could be a lifelong career. The Roman army was originally made up by drafted peasants and citizens, but with General Gaius Publius Marius reforms in 107 BCE it was now possible to join the army and become a professional soldier. A reform of Rome’s military was at that time necessary due to new enemies and the continuing expansion of the Roman Empire. For the Roman citizens it was a welcoming opportunity for steady work and income, in a world where the use of slaves had made many ordinary citizens unemployed. The professional armies were also a political, peace-building and diplomatic tool like modern armies are today. In addition to being a tool for new conquests and steppingstone for the advancements of political careers of generals and upcoming emperors.
The army consisted of legions and was not a single organized national unit. Romans spoke of “the armies” - Excercituus in plural. And the soldiers as Milites - individuals hired by their Legatus - General. The legions were raised in connection with actual wars, campaigns or threads. Often financed by private means, by the generals they served. This “private army” system changed in principle with the reforms of Augustus. Who had seen the rise of demagogues like Sulla, Pompey and Caesar. In classical roman (Augustus’) eyes, they were renegades who forced their will on Rome, because of their ability to raise and sustain private financed legions.
The Imperial legions therefore became “nationalized” with reference to the Emperor himself. (But still not to the state). The legions never saw themselves as part of the state. A strong independence lingered and loyalty from the milites should be earned by their leaders. The historian Flavius Josephus describe it very well in his “De Bello Judaico” - the Jewish War - how Vespasian worked hard through negotiation and diplomacy to gather loyalty from legions and fleets, before he could make his move on being Emperor instead of Vitellius in 69 AD.
The Imperial Legionnaire was a professional soldier and not a warrior. His personal interest was to survive the fight, not to be a hero. A fight he had prepared for through hard training and a strong feeling of loyalty to his Fratres - brothers in arms. Therefore, he used a lot of resources on equipment, which protected him from head to toe. His loyalty lay with his immediate superiors, and the legion was his family. The only way a Roman man could break his father's right to rule over him, was to become a soldier. This surpassed the right to warrant from the father to the officers in the legion. The core of Roman society was family, with the Pater Familias as head of the family. He had absolute control over all members of the family and this structure was reflected onto the legions.
The smallest unit in a Legion was called a Contubernium - in modern terms equal to a section/group. It consisted of 8 men for every tent. The tent - Papillio - was a low leather tent with a floor area of just around 3 x 3.5 meters. In addition to the tent the Contubernium also shared equipment for cooking and a mule that could carry the camp equipment during marches. A leader of the group - a Decanus - was chosen among the men. Often the mule was led by a slave - a Calo - that served the Contubernium.
The Contubernias was assembled in Centurias - in modern terms equal to a company sized unit. 10 Contubernias to a Centuria, making the Centuria consist of 80 men.
The Centuria was led by Centuriones (junior commissioned officers - ranking from second lieutenant to major) and Optiones (non-commissioned officers). Each Centuria also had its own signaling staff; The Signifer, who wore the centuria Signum (Standard/colours) and the Cornicen (hornblower). A Tesserarius (a bookkeeper) was also among the centurias NCO's.
Above the 6 junior ranks of centurios the army consisted of senior ranking officers from tribunes to Legatus - modern day lieutenant colonels to generals.
Six Tribunes - one Tribunus Laticlavius (the broad striped), of senatorial rank, and 5 Tribuni Augusticlavii (the narrow striped) from the class of equites - knights - in Rome, was responsible for the Cleric Work of the Legion. The hands on military advisory for the Legatus - the general, was the job of the Pilus Primus, the head Centurion of the legion.
Until the time of Augustus the legions were mainly assembled in Manipels (Handfuls). The Manipels were double-Centurias of 120 men. Each centuria of 60 men. A legion was fully assembled at 4800 men.
In the imperial legion, the centurias were assembled in 6 Cohorts consisting of 10 centurias of 80 men. First Cohort though, in double-centurias of 160 men giving the legion, fully assembled a count of 5400 men.
Often other unit sizes were organized than Cohorts and Legions, these units were assembled in Vexillias - “Flag units”.
The legions mainly consisted of infantry, but normally there was also a cavalry unit attached, an Ala (wing) consisting of around 330 men. The Ala was divided into 12 subunits called Turma led by Decurions.
Often Auxilia cohorts were attached to the legion in their own independent cohorts. Auxilia was none-roman support troops mainly consisting of none infantry such as cavalry, archers, stone slingers, marines etc.
The Augustan reforms also changed the length of duty for the legionnaires. In the former army, service was of 10 to 16 years. The “Augustan contract" was 20 - 25 years and 5 years of Evocatus - veteran.
The victorious Legions
On the battlefield the roman legions were almost invincible. Primarily due to their hard training and high morale. It was an honour to fight and to win. But a main factor was also the high mobility that the organisation in the legions gave on the battlefield. The greek Phalanx system that had dominated the battlefields for centuries became obsolete, facing the mobility of the new roman system with Centurias, Maniples and later Cohorts. This new system, used already in the battle of Sentinum 295 BC, gave the roman commanders a mobility that can only be compared with the german army of 1939, and the “blitzkrieg” strategy. It was a tactical revolution that made way for an empire.